Erosion and Deposition
Erosion is the process by which earth materials are
moved by natural agents like water, wind, and ice.
Running Water
Running Water is the most effective agent of erosion.
The sun is where running water gets its energy
Rocks are weathered both Chemically and Physically by running
water
Physical
Abrasion is the term given to the use of sand, pebbles,
and even boulders as cutting tools to grind away at the
stream bed. During this process the “tools” themselves
wear down.
Chemical
The water dissolves soluble minerals
Rivers carry rock material in three ways
Solution – This is material that is dissolved from the
bedrock. Most commonly found in solution are compounds
of calcium and magnesium.
Suspension – When small rock particles, such as clay silt
and fine sand, are kept from sinking by the turbulence of the
stream. This gives the water a muddy look.
Bed Load – Sand, pebbles, and some boulders which move
along the stream bed.
Carrying Power
Carrying Power is indicated by both the total amount of
sediment in a stream and by the size of the particles being
moved.
**The stream discharge and speed will determine the
carrying power of the stream.
Discharge is the volume of water flowing past a given
point at a given time.
Speed is generally determined by the steepness, or
gradient, of its bed.
A stream moving at high speed with a high discharge
can carry much larger sediments then a slow moving
stream. Example: Spring time snow melting and
excessive rain
The River Valley
**Rivers tend to have a V-shaped
valley because the tend to flow at
high speeds and dig into the
stream bed.
Base Level is the lowest level a river can cut into its bed.
To form a permanent stream rain water must flow down a
slope and dig deep enough to cut into the water table.
This wearing away of the land to form a stream valley is
called headward erosion.
A Divide is an area of high
land that separates one river
valley from another.
On either side of a divide a river
system may form.
Watershed is all of the land that
drains into the river either directly
or through its tributaries.
Waterfalls
Water flowing over a Steep cliff
will result in a waterfall. Waterfalls
are not permanent structures.
Undermining is the erosional
process occurring at the base of a
waterfall. Here water carrying
sediment plunges down and back
into the the stream bed and cliff
below. This causes the rocks at the
top of the falls to overhang. Over
time this overhang will collapse and
the stream will move back towards
its source
River Deposition
Deposition occurs when a stream either
decreases in speed or discharge.
Generally the speed decreases when its slope decreases
or its bed widens. The greatest loss of speed occurs
when a river empties into a quiet body of water.
A decrease in discharge would occur if a river traveled
through an area with low precipitation.
As rivers begin to decrease their slope they
move slower and will begin to move side to side.
As the valley wall on either side is eroded the
valley floor is widened.
A Flood Plain is the widened valley floor area
which will accumulate water during times of
excess rain when the river floods
Erosion and Deposition in a River
Yukon River Basin
Meanders are broad curves in the river
(each bend or turn)
Erosion is greatest on the outside of a
meander where water is flowing the
fastest. (cut bank)
Deposition is greater on the inside of the
meander where the water flows slower.
(fill bank)
Oxbow Lake - Meanders can only become so large
before they break through into another meander. The
river then deposits mud and silt along the end of the
abandoned meander . The now separated meander
becomes a lake.
Running Water Deposits Well-Sorted Particles
Vertical Sorting – When
sediments are suddenly
deposited into water. The
particles separate by size with
the largest on the bottom and
smallest on top.
Horizontal Sorting – When rivers empty their sediments into
quiet bodies of water. Particles are sorted by size with larger
particles being found closer to the shore and smaller particles
being carried out into the body of water to be deposited.
Delta – A fan-shaped deposit of sediment
at the mouth of a river
River Vocabulary (see attached)
River Stages
Young River
Slope of the land – Steep
Velocity – Fast
Type of Erosion – Headward Erosion
(deepens valley)
Shape of the Valley - Straight
Characteristic Features – Channel
touches both valley walls
**Valley is V-Shaped
Examples – Upper Hudson and Niagara
River
Special Features – Waterfalls, rapids,
islands, no tributaries
Mature River
Slope of the land – Less Steep
Velocity - Slowing Down
Type of Erosion –Lateral Erosion
(widening the valley)
Shape of the Valley - Meandering
Characteristic Features – Channel
touches both valley walls on the meanders
Valley is wide and flat
Examples – Ohio and Mississippi Rivers
Special Features – Oxbow Lake,
meanders, flood plain, tributaries
Old River
Slope of the land – Nearly Flat
Velocity – Sluggish
Type of Erosion – No Erosion (depositing sediments)
Shape of the Valley – Very Meandering
Characteristic Features – Channel does not touch the
valley walls and valley is very wide and flat
Examples – Lower Mississippi
Special Features – Flood plain, oxbow lakes, yazoo
streams, backswamps (bayous)